This invention relates to a wear resistant iron alloy article formed by compacting a predominantly iron powder mixture containing carbon powder and a boron source and sintering under conditions that cause the carbon and boron to diffuse into the iron to form dispersed, hard borocementite particles. More particularly, this invention relates to a presinter treatment to expel contaminant oxygen from the compact to enhance carbon and boron diffusion and to thereby increase borocementite formation.
U.S. Pat. No. 4,678,510, issued to Jandeska et al in 1987, describes an iron alloy article characterized by hard borocementite particles dispersed in an iron alloy matrix to enhance wear resistance. The article is produced by compacting and sintering a powder mixture composed predominantly of low-carbon iron powder and including carbon powder and a liquating diffusible boron source. A preferred boron source comprises a combination of nickel boride powder and iron boride powder. An expendable lubricant is included to facilitate compaction and is vaporized at a relatively low temperature, typically not greater than about 500.degree. C., during the early stages of the sintering cycle. Sintering is typically carried out at a temperature between about 1100.degree. C. and 1120.degree. C. to bond the iron into an integral structure. During sintering, carbon diffuses into the structure to form a mainly pearlitic or martensitic matrix. The boron additive forms, within the environment of the compact during sintering, a transient liquid phase, whereafter boron diffuses into the iron and combines with carbon to produce the desired borocementite particles.
Although the borocementite particles form a hard phase that substantially improves wear resistance, it is desired to minimize the boride additions no only to minimize the cost associated therewith, but also because the brittle boride reduces compressibility of the powder mixture. This increases the compaction force required to produce a green compact having adequate strength for handling and accelerates die wear.
Sintering is preferably carried out in a vacuum to avoid oxidation of constituents, including in particular the boron, since boron oxide does not suitably relinquish boron for diffusion. It is also found that boron oxidation interferes with carbon diffusion, particularly noticeable within interior regions of large compacts, resulting in the presence of carbon-deficient, relatively soft ferrite grains that reduce mechanical properties of the product. U.S. Ser. No. 161,518, filed by Jandeska et al on Feb. 29, 1988, describes an addition of an agent comprising an oxygen getter, preferably iron titanium alloy powder, to promote carbon and boron diffusion and thereby produce a more uniform product microstructure. While the addition of an oxygen getter satisfactorily inhibits boron oxidation, the addition increases cost and further reduces compressibility. Thus, it is desired to minimize, or even possibly eliminate, the oxygen getter addition without compromising carbon and boron diffusion.
It has now been found that a major source of oxygen within the compact during sintering is iron oxide that contaminates surfaces of iron particles. As the evacuated compact is heated to sintering temperature, the iron oxide dissociates, releasing oxygen for reaction with boron. In addition, exposure of boron to the oxygen, and thus the susceptibility to oxidation, is increased by the formation of the liquid phase during sintering.
It is an object of this invention to provide an improved method for manufacturing an iron alloy article by compacting and sintering a predominantly iron mixture comprising carbon and boron additions, which method comprises a presinter treatment to reduce oxygen contamination within the compact and thereby enhance carbon and boron diffusion during sintering.
It is a more particular object of this invention to provide a presinter treatment carried out prior to sintering a predominantly iron powder compact comprising carbon powder and a liquating diffusible boron source to form an iron alloy product characterized by dispersed, hard borocementite particles that enhance wear resistance, which presinter treatment is effective to decompose iron oxide contaminant within the compact and expel the unwanted oxygen released thereby prior to activating the boron source and thus promotes more efficient carbon and boron diffusion during sintering as evidenced by increased borocementite formation and a more uniform matrix microstructure. As a result, the amount of boron source in the powder mixture may be reduced, improving compressibility, extending die life and ultimately reducing raw material and processing costs. In one aspect of this invention, the presinter treatment permits the amount of oxygen getter additive to be reduced, or even possibly eliminated, further improving compressibility and reducing cost.